1. Field of the Invention
The invention relates to a data transmission apparatus applied to the recording of a digital audio signal.
2. Description of the Prior Art
As one apparatus for recording/reproducing a digital audio signal, there is a known VTR (8-mm VTR) which divides one track scanned by a rotational head into a recording section for a video signal and a recording section for a digital audio signal and records a video signal of one field and a time-compressed digital audio signal of one field in the two recording sections, respectively. In the 8-mm VTR, the system by which an analog audio signal is FM modulated and recorded by the superimposition on a video signal is called a "standard recording system", and as an option, a recording format of the above-stated audio signal is standardized. Such an apparatus for recording a digital audio signal is disclosed in the U.S. Pat. No. 4,542,419.
In a digital audio signal of an 8-mm VTR, the sampling frequency is to have 2 fh (fh: horizontal frequency), and the quantization bit number is selected to be 8 bits. These sampling frequency and the quantization bit number do not provide satisfactory values in terms of high quality audio reproduction. Consequently, it is thought that a digital audio signal having a sampling frequency of 48 kHz and a quantization bit number of 16 bits should be recorded and reproduced. In this case, one problem is that the transmission data amount increases and the recording wavelength on the magnetic tape becomes short.
Especially, in the 8-mm VTR, it is difficult to achieve an increase of the transmission data amount in every kind of magnetic tape because of the use of recording sections of short track length. Specifically, although digital audio data having the above-mentioned high quality can be recorded and reproduced using a metal evaporated tape, there is a problem that the recording/reproduction cannot be performed using a metal coating tape.
To solve this problem, it has been suggested that the bit number of one sample of a digital audio signal could be changed depending on the kind of magnetic tape to be used. For example, an analog audio signal could be digitized with a quantization bit number of 16 bits. Compression processing of this digitized audio signal is not carried out in the case where metal evaporation tape is used for recording and reproduction, while 16-bit-to-12-bit compression is done in the case where the metal power coating tape is used. Therefore, when such a signal compression is done, the number of samples contained in one field of NTSC system or CCIR system becomes a predetermined number to enable the bit number of one sample to be compressed, thereby lowering the data transmission rate.
To cope with an error occurring at the time of the recording/reproduction, there is a known error correcting method in which digital data contained in one field period, for example, is arranged in two-dimensional form, and encoding using error correction codes is performed in the first direction and the second direction of the two-dimensional array (called a "block"). Changing the bit number of one word depending on the tape used creates the necessity of making different block sizes and requires two kinds of encoders and decoders for error correction encoding and decoding corresponding to the two kinds of bit numbers, respectively. However, because this increases the hardware size consequently, it is desired that the encoder and the decoder can be each commonly used to the greatest possible extent to cope with the two kinds of PCM audio signals.
As described in the specification of the U.S. application Ser. No. 337,617, the applicants of the present application have proposed a system in which four-bit dummy data (specifically "0" data) is added in the compression of 16 bits/word to 12 bits/word, 12-bit data is converted into pseudo-16-bit data, the sizes of blocks are made equal and the converted data is transmitted after processing at an encoder with the dummy data eliminated. In this system the encoder and decoder can each be commonly used between 12 bits and 16 bits.
Additionally, as a system in which the sizes of blocks of error correction codes are kept equal between 16-bits data and 12-bits data, the one described in the U.S. Pat. No. 4,688,225 and in the U.S. Pat. No. 4,758,907 is known. This system has been adopted in the DAT (digital audio tape recorder). However, in the case of the DAT, the bit number is reduced by 3/4 from 16 bits to 12 bits, and the sampling frequency is lowered by 2/3 from 48 kHz to 32 kHz. As a result, the data amount is decreased by 1/2, while the relative speed between the rotational head and the magnetic tape is reduced by 1/2. The recording wavelength is kept equal between the 16-bits data and the 12-bits data. For this reason, as mentioned above, the DAT system described above is not applicable to the case where the recording wavelength is varied to select a most suitable recording wavelength depending on the type of magnetic tape used.
The present invention relates to an improvement of the previously proposed system where the sizes of blocks processed at an encoder of an error correction code are made equal by adding dummy data with respect to data having different bit numbers as disclosed in the U.S. application Ser. No. 337,617. In the previously proposed system, dummy data and a digital audio signal are mixed in one symbol (or word) which is a unit of the encoding. In addition, when the encoding for the error correction code is performed with respect to a block with dummy data added, both of the dummy data and the digital audio signal coexist in the set of error correction codes. Further, one error correction code is suitable for data arranged in the diagonal direction of the block, and the encoding of the other error correction code (C1 parity) is performed for a redundant code (C2 parity) of one error correction code.
Consequently, although it is possible to transmit data except for dummy data after the encoding for error correction and to finally magnetically record that data, it is impossible to exclude a redundant code caused by data including the dummy data. For this reason, the reduction of the amount of data to be transmitted is not sufficient. As a result, there is still a difficulty in the case where the compressed PCM signal of 12 bits is recorded onto metal powder tape.